The present invention is directed to ink compositions. More specifically, the present invention is directed to aqueous ink compositions exhibiting rapid drying times that are particularly suitable for use in ink jet printing processes. One embodiment of the present invention is directed to an ink composition which comprises an aqueous liquid vehicle, a colorant, and a phosphate ester.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles necessary for high resolution printing, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies, and also decreases printing speed. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. Nos. 4,601,777, 4,251,824, 4,410,899, 4,412,224, and U.S. Pat. No. 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Inks suitable for ink jet printing processes are known. For example, U.S. Pat. No. 4,695,846 (Suzuki), the disclosure of which is totally incorporated herein by reference, discloses an ink jet recording process for recording colored images using at least two kinds of inks different from one another in dye concentration to express one color, wherein the inks are toned so that hues given by the inks are continuous. The inks employed for the disclosed process generally comprise a dye and a solvent generally composed primarily of water and a wetting agent such as a glycol or other water-soluble organic solvent. The inks can also contain a surfactant. Japanese Patent Publication 56-49771 discloses inks for ink jet recording that comprise water soluble direct dyes, glycerol and optionally other polyhydric alcohols, N-methyl-2-pyrrolidone, and a surfactant which has a surface tension of 30 dynes per centimeter or less at the micelle concentration in water. In addition, U.S. Pat. No. 4,165,399 (Germonprez) discloses binderless ink compositions suitable for use in ink jet printing operations on polymeric resin surfaces to form images which are highly resistant to abrasion and to steam pasteurization conditions by virtue of penetration of the colorant into the sub-surface structure of the resin. The inks generally contain a basic or neutral dye, a water and alcohol solvent system, a surfactant, an aliphatic ketone or ester, and optionally an aliphatic hydrocarbon of 8 to 12 carbons.
Further, U.S. Pat. No. 4,229,747 (Hwang), the disclosure of which is totally incorporated herein by reference, discloses an improved fast drying jet printing ink for use in jet drop printers and copiers which comprises an aqueous solution of a water soluble or solvent soluble dye, an organic chemical solvent comprising a glycol ether having 8 to 12 carbon atoms, a bridging agent for solubilizing the solvent and dye in solution comprising an ethoxylated higher alcohol amine-amide, a humectant, and a defoaming agent. In addition, U.S. Pat. No. 4,259,675 (Mansukhani) discloses an ink composition for use in continuous stream jet printing processes comprising an aqueous solution of a water soluble dye and a humectant consisting of at least one member of the group consisting of organic phosphonates and organic phosphites. U.S. Pat. No. 4,421,559 (Owatari), the disclosure of which is totally incorporated herein by reference, discloses an aqueous fluid ink suitable for use in an ink jet printer which includes urea, a water-soluble dye, water, and a humectant selected from aliphatic polyhydric alcohols and alkyl ether derivatives of the aliphatic polyhydric alcohols. The viscosity is maintained between about 2 and 3 centipoise at 20.degree. C. and the ink is made quick drying by adjusting the pH to between about 12 and 13.5. Other publications disclosing ink jet ink formulations include IBM Technical Disclosure Bulletin, vol. 25, no. 2, page 518 (July 1982), IBM Technical Disclosure Bulletin, vol. 24, no. 2, page 920 (July 1981), and IBM Technical Disclosure Bulletin, vol. 18, no. 4, page 1099 (September 1975).
Other ink compositions are also known. For example, U.S. Pat. No. 2,324,550 (Wolfe) discloses a printing and lithographic ink containing a high percentage of solids and exhibiting rapid drying characteristics. The ink is designed for application to metal and similar non-absorbent surfaces at relatively high baking temperatures and comprises a vehicle consisting of a varnish type synthetic resin, a rapid drying and plasticizing oil of high iodine value, and a tri-alkyl phosphate, as well as a color pigment incorporated in the vehicle. The tri-alkyl phosphate functions as a solvent having the characteristic of rapid evaporation when the ink is baked at relatively high temperatures to leave a tenaciously adhesive ink film on the metal surface. In addition, U.S. Pat. No. 4,001,164 (Chu) discloses a fast drying ink which contains a colorant, a non-volatile carrier, and a film forming material of the structure ##STR1## wherein A is a segment, non-polymeric or oligomeric, bearing one or more functional groups, Z is an organic linking radical, B is an oligomeric or polymeric segment, and m and n are 1, 2, or 3, the total not exceeding 4.
U.S. Pat. No. 2,128,946 (Katzman) discloses a tetraphosphate of an aliphatic polyhydroxy substance wherein the hydrogen of at least one hydroxy group of the aliphatic polyhydroxy substance is replaced by a radical selected from the group consisting of alkyl and acyl radicals. The material is useful as an interface modifier, to increase the oiliness of lubricating oils and greases, to act as an emulsifying agent, to reduce viscosity, and the like, and may be added to a bath containing an aqueous medium with or without additional substances, including dyes. In addition, U.S. Pat. No. 2,052,029 (Harris) discloses a phosphoric acid ester of a polyhydroxy substance having not less than four esterifiable hydroxy groups but not including hexahydric alcohols, the hydrogen of at least one of the hydroxy groups being replaced by a lipophile radical having at least four carbon atoms. The material is useful as an interface modifier, and may be added to a bath containing an aqueous medium with or without additional substances, including dyes.
Although known compositions are suitable for their intended uses, a need continues to exist for ink compositions suitable for use in ink jet printing processes. In addition, a need remains for ink compositions particularly suitable for thermal ink jet printing processes. Further, there is a need for ink compositions that exhibit rapid drying times. There is also a need for ink compositions that have low viscosity (generally no more than about 5 centipoise) and exhibit rapid drying times of less than about 15 seconds when used in thermal ink jet printing processes to generate images on plain paper. Additionally, there is a need for ink compositions that enable ink jet printing processes with increased output rates. Further, a need remains for ink compositions that enable rapid ink jet printing processes on paper substrates with no need for drying means such as heat, forced air, or the like. In addition, there is a need for ink compositions of low viscosity that enable the generation of high quality prints, particularly when used in thermal ink jet printing processes.